1. Field of the Invention
The present invention relates to devices for separating debris particles and gas from fluids in machinery, such as lubricants in an engine; and more particularly to such devices that perform the separation by creating a fluid vortex.
2. Description of the Related Art
Modern turbine engines, such as those used in aircraft, are lubricated by oil supplied to moving engine components by a pump that draws the oil from a reservoir. The oil flows from those components into sumps within the engine from which scavenger pumps force the fluid back to the reservoir. In the course of flowing through the engine, the oil often picks up metal and non-metal debris particles and also becomes aerated due to a turbulent flow. Therefore, it is common practice for this mixture to pass through an apparatus that separates the particles and entrained gas from the lubricating oil prior to entering the reservoir.
Such separation has conventionally been performed by a three-phase cyclonic separator, such as the one described in U.S. Pat. No. 6,348,087. With reference to FIG. 1, this type of separator receives the fluid mixture from the engine via an inlet passage 100 that is tangentially aligned with the curvature of the inner wall 102 of a cylindrical chamber 106. This alignment causes the fluid to travel in a vortex 108 downward into an annular debris collection area 110. The centrifugal force of the vortex drives the heavier debris particles outward and downward against the cylindrical inner wall 102 and into the debris collection area while the fluid flowed through a centrally located outlet 104. The tangential velocity of the circular flow drives the debris particles into a linear exit passage 112 that extends tangentially from the curved surface of the cylindrical inner wall 102 in the debris collection area. A magnetic particle collector 114 was located at the remote end of the exit passage to retain metal particles. The particles must travel some distance along that exit passage 112 before reaching a magnetic particle collector 114. Therefore upon entering the exit passage, the particles were required to have enough momentum to reach the magnetic particle collector. Small particles often did not possess sufficient momentum and thus were not retained by the collector.
Specifically, upon entering the exit passage, the particle was out of the rotational force field of the vortex. The primary forces counteracting the particle motion were gravity and drag forces. The drag force Fd is given by the expression:
      F    d    =                    C        d            ⁢      ρ      ⁢                          ⁢              AV        2              2  where Cd is the drag coefficient, ρ is the transport fluid density, A is the projected area of the particle in the direction of flow, and V is the particle velocity which is assumed to be equal to the fluid velocity.
The settling velocity VS of the particle follows Stokes law and is defined by the equation:
      V    s    =                    gd        P        2            ⁡              (                              ρ            p                    -                      ρ            f                          )                    18      ⁢                          ⁢      µ      in which g is the earth gravitational force, dp is the particle's primary dimension, ρp is the particle's density, ρf is the density of the transport fluid, and μ is the fluid viscosity.
The drag force acts against the particle's momentum, while the force of gravity moves the particle normal to its intended trajectory. The attractive force of the magnetic collector is not apparent until the particle is relatively close due to the design of the pole piece that confines the flux lines to a small envelope. Therefore, the particle must possess sufficient kinetic energy to sustain the dissipation of the drag force and reach the perimeter of the magnetic influence. The gravity force and settling velocity for small particles is insignificant for the brief particle transport period (typically <150 milliseconds) and in this model are disregarded.
It is desirable to have a small a fluid pressure drop between the separator inlet and lubricant outlet as possible. However, the pressure drop is directly proportional to the flow rate of the fluid and thus the tangential velocity of the circular flow. In other words, as the pressure drop is reduced so too is the tangential velocity of the fluid flow which drives the particles from the cylindrical chamber into the collector exit passage. This relationship limits the physical size (diameter) of the separator and thus the amount of fluid flow there through. As a consequence, enlarging the diameter of the separator chamber to accommodate a greater fluid flow reduces the tangential force of the fluid flowing through the chamber and the ability to separate out the particles.
Therefore, it is desirable to improve the debris transport efficiency of the separated particle from the chamber wall to the debris collection site in order to provide a cyclonic fluid separator that can efficiently operate at greater fluid flow rates.